Substrate processing, such as semiconductor wafer processing, can raise the temperature of the substrate above an optimal processing value. For example, substrate heating during physical vapor deposition (PVD) can cause residual thermal stress in a deposited film, lead to poor adhesion of the deposited film, and otherwise damage sensitive substrates.
Some substrate processing tools use batch processing, whereby semiconductor wafers are transported on pallets through the tool and processed while held by self-contained wafer holders. For example U.S. Pat. Nos. 6,530,733, 6,682,288, and 6,821,912, the disclosures of which are incorporated herein by reference, describe substrate processing pallets for batch processing, related substrate processing machines, and methods in which the pallets are exposed to various temperatures during processing.
In general, the temperature of substrates processed on a pallet depends on the processing power and the degree of thermal contact between the substrate and the pallet. Temperature control can be accomplished by reducing the processing power, but this reduces the tool throughput, which is not desirable because it increases the processing cost per substrate. It is preferable to control temperature by maintaining sufficient thermal contact between the substrate and the pallet.
Semiconductor processing is often accomplished at low pressures in a vacuum chamber. Films deposited by PVD, for example, are often deposited at gas pressures of a few mTorr. At these low pressures, the thermal contact between surfaces is often low, and may not provide sufficient cooling for high throughput processing.
Substrate Cooling Principles:
Several general principles can maximize substrate cooling on a pallet. These include using pallet materials with high thermal conductivity, ensuring that the substrate and pallet are parallel and in good physical contact, decreasing the roughness of the surfaces, increasing the pallet heat capacity and increasing the gas pressure at the interface.
Materials with good thermal conductivity include various metals and ceramics. Good physical contact can be maintained by having flat, parallel substrate and pallet surfaces and by increasing the pressure of the substrate onto the pallet. Decreasing the surface roughness is accomplished by use of smooth polished surfaces, since smooth surfaces typically have twice the solid spot conductance of rough surfaces for the same pressure of substrate onto pallet. Increasing the heat capacity of the pallet improves cooling, because for the same process heat loading, the equilibrium tray temperature remains cooler.
Increasing the gas pressure at the interface increases substrate cooling, because heat transfer across a gap is primarily due to gas conduction, which is essentially a linear function of gas pressure under typical processing conditions. Increasing gas pressure at the interface to facilitate cooling is known as active cooling using backside gas pressure. In contrast, employing smooth surfaces to facilitate thermal conduction and cooling is known as passive cooling.
The substrate can be clamped against the pallet by mechanical or electrical means to increase the gas pressure at the interface. Clamping is necessary because otherwise the gas can either escape or possibly lift the substrate off the pallet. The electrostatic chuck (ESC) is an electrical method that permits uniform holding over virtually the entire substrate area and avoids edge exclusion and particles associated with mechanical clamps.
Pallets with Passive Cooling:
The simplest pallet consists of a tray fabricated from a single block of metal, such as aluminum. An improved surface smoothness can be achieved by bonding ceramic or semiconductor pads to the metal pallets to act as the interface between substrate and pallet. Silicon wafers can be used as interface pads because they are relatively inexpensive and are typically have sub-micron surface roughness resulting from chemical-mechanical polishing (CMP). A surface polished with CMP is much smoother than a metal pallet, with a resulting increase in substrate cooling. Such a tray is available as Part No. K11007815: Aluminum Silicon Tray Assembly, from NEXX Systems, Inc. of Billerica, Mass.
There are significant limitations to the substrate cooling ability of passive-cooling pallets made from either single metal block or containing ceramic or semiconductor interface pads. The surface finish and the flatness that can be achieved cost-effectively using standard metal fabrication techniques limit the cooling of simple metal pallets. Pallets containing ceramic or semiconductor pad interfaces are not robust because of thermal expansion mismatch between the metal pallet and the ceramic or semiconductor. As listed in Table 1, silicon has a thermal expansion coefficient approximately eight times smaller than the pallet's aluminum base metal. When such pallets are heated, the higher thermal expansion of the metal pallet can cause cracking of the pad, creating a rough surface and requiring expensive pad replacement.
TABLE 1Thermal properties of materials used in substrateprocessing pallets over the 20° C.-80° C.pallet temperature use range. Values for aluminumare for a range of alloys. Values for alumina/PTFEare for a range of commercially available compositions.Thermal ConductivityThermal ExpansionInterface Pad Material(W/cm° C.)Coefficient (ppm/° C.)Aluminum1.5-2.421-25Silicon1.2-1.82.6-3.2Alumina/PTFE 0.01-0.02520-30compositePallets with Active Cooling:
A pallet can incorporate an electrostatic chuck (ESC) and active backside gas (BSG) cooling to facilitate cooling. Similar to capacitors, electrostatic chucks can hold a charge for a period of time after being disconnected from their power supply. The ESC self-discharge time depends on its material of fabrication and geometry, which determine the electrical resistivity and the ESC capacitance. Self-discharge times for ceramic or polyimide ESCs are shorter than the several minutes typical of semiconductor processing operations. This requires the ESC to be continuously connected to an energy source during processing.
Furthermore, pallets are transported through a tool, which prevents continuous electrical or gas connections. Instead, the connections are made and re-made as the pallet moves through the system. The electrical connections to the pallet can still be protected from plasma and arcing, just as in a fixed system, but in a way that allows for disconnection and re-connection.
Pallet Cleaning:
Pallets in a plasma processing environment require periodic cleaning. For example, thin films deposited on pallet areas not covered by substrates can be cleaned when the deposits exceed a certain thickness. A pallet can be removed from the tool and cleaned using mechanical or chemical means. Conventional pallets are often cleaned using grit blasting with abrasive media. However, such cleaning can harm delicate pallet features or embed abrasive media in areas from which they are difficult to remove. Chemical cleaning of the entire pallet is a preferable alternative to grit blasting, but pallets often contain adhesives or other polymeric materials which can be harmed by harsh chemicals used for cleaning.